Impedance matching coupling system



, U] April 11, 1939. w. VAN B. ROBERTS I 2,153,776

I MPEDANCE MATCHING-COUPLING SYSTEM Filed Feb. 7, 1956 VINVENTOR WALTER U4 8.8086'05 ATTORNEY V 4 ance. If this resistance is then found to be too When the'value of the capacitor K is adjusted l5 Patented Apr. 11, 1939 y 5 2,153,776

UNITED STTEIS orris IMPEDANCE MATCHING COUPLING SYSTEM Walter van B. Roberts, Princeton, N. J., assignor' to Radio Corporation of America, a corporation of Delaware Application February 7, 1936, Serial No. 62,784

6 Claims. (01. 178-44) This invention relates to impedance matching a resistive load to the output of the network. It coupling systems and has particularly to do with will be observed that the choke coils 4 and 9 and networks of that character where used in radio the blocking condenser B are required in order to transmitting and receiving apparatus. One of permit suitable energizing of the tube electrodes, the objects of my invention is to provide a but that this makes the arrangement undesirable. 5 means and a method whereby proper adjustment According to the present invention the arof impedances between stages of a radio frerangement of Fig. 2 has been found to give satisquency amplifier may be effected, thus enabling factory service and without sacrifice of any of the the amplifier to be operated at optimum efadvantages of the system shown in Fig. 1. The 10 ficiency. It is another object of my invention principal advantage to be derived from the cirto provide a system which is relatively free from cuit of Fig. 2 is that no choke coils or blocking t e o c e c0118, blocking fiend-cheers and condensers are required. This circuit looks like the like. a pair of tuned coupled circuits of the ordinary y i n i n wil e best n rs p n variety, but upon further scrutiny of the diagram erence to the acc mpanyi d aw n n W in the light of the discussion to follow, it will be F ure 1 is a diagrammatic Showing of a seen that a number of important advantages are cuit which is referred to in the following discust b d riv d,

sion of the theory of operation of my invention; In t figure parts i i t th of Fig 1 and are given like reference characters. However, the Fi 2 illustrates diagramma cal y a p e e elimination of choke coils and blocking con- 20 circuit arrangement of the invention. densers results from using a coupling transformer Re e n ir o I Sh W a P 0f p l2. The primary L of this transformer is fier tubes l a d Coupled by What is essentially shunted by a capacitor K, whereas the secondary a well known impedance matching network, 1," is shunted by acapacitor c, sometimes called a p neiiwork- The anode 3 It is important that the coupling of the trans- 25 of the tube l is fed with suitable direct cu re former i2 should be made very much closer than anode potential through a choke c011 The nby any value ordinarily used in tuned networks, P energy pp 1J0 theglid 5 is amplified and but not so close as would be obtained in an ideal p ess d a oss the "Capacitor also across an transformer. The capacitors are not used to tune inductive impedance L1 to the grid 7 of the tu e. the coils but to match the impedances; The 2, The anode 8 of this tube feeds Output e y method of adjustment is quite similar to that to any suitable utilization device w ch y of hereinabove described in respect to Fig. 1, that is course, be a further Sta e of ampllfiqatlon an to say, the value of the capacitor C is first made antenna system if desired. The coupling network Very large, then the Value of the capacitor K is may be adjusted by means of capacltor K on adjusted to make the input impedance a pure the input sidev of the inductive impedance L1 and resistance Assuming that this resistance turns i f g ga gg i i gig gggg zgi fgfizgg' cut to be larger than desired then the next step g is to reduce the value of the capacitor C and f b si tential source l0 in circuit i 2 2 56 2? p0 subsequently to read ust the Value of the capaci- In using the circuit of Fig 1, the capacitor C tor K. This operation is repeated until the input is set first to a relatively large value and then resistance 13 of the desired Valuethe capacitor K is adjusted so as to make the load The mathematical basis making e adimpedance i t hi h t be rk a pure r istjustments as aforesaid will now be considered.

large for efiicient energy transfer from the source to make the input impedance of the network of current then the capacitor C is adjusted to a shown in Fig. 1 a pure resistance then the input smaller value and the capacitor K readjusted to Conductance of thi tw rk ha a value of again give a pure resistanceloading. This opera- R tion can be repeated until the right value of the H m capacitor C is'obtained for giving the desired wzLlzi'RzuwwzcLoz loading of tube where l/R is the conductance of the utilization The circuit as shown in Fig. 1 may be used circuit into which the network delivers energy. either in class B or in class C amplifiers. In correspondingly when the capacitor K, as

either case there is a grid current which presents shown in Fig. 2, is adjusted to make the input of g5 the network a pure resistance then the input conductance of this network has a value k R w L+ 12 (1 awn) where k is the coefficient of coupling between the primary and secondary coils of the transformer l2 and Le=L(1-Ic and L=L=L.

It is not essential that the primary and secondary coils of the transformer 12 should be equal, but the equations are thereby simplified. If they are equal then the input conductance as given by the above expression in respect to Fig. 2 is obviously identical in form with the expression for the input conductance expressed in reference to the network of Fig. 1. If we choose the values of k and L, respectively, so that Le=L1 then the two expressions have the con stant ratio k Referring now to Fig. 2 and to the general case where the two coils are not necessarily equal, the load R will be properly matched to the internal resistance G of an alternating current source of a given frequency, as for example, the discharge tube I, when the capacitors C and K are chosen to satisfy the following equations:

It will be appreciated by those skilled in the art that my invention can be applied in numerous forms of radio circuits and its scope is therefore limited only in accordance with the definition of the claims.

I claim:

1. A network for converting an alternating current load impedance having conductance 1 /R into a pure resistance of a desired value G comprising a secondary condenser connected across said load to make a total effective capacity C2 across the conductance of said load, a secondary coil of inductance L2 connected across said load, a primary coil of inductance L1 arranged to be inductively and relatively closely coupled to said secondary coil, and a primary condenser connected across said primary coil to make a total capacity C1 across said primary coil, said capacities being chosen to satisfy the equations H in claim 1 in which the coefficient of coupling between the primary and secondary coils is at least 15 percent. and not greater than 80 percent.

3. An impedance matching network as defined in claim 1 in which the coefficient of coupling between the primary and secondary coils approaches 40 to 50 percent as an optimum value.

4. A reactance network for matching the impedance of a load to that of a source of alternating currents, comprising a variable capacitor 0 connected across the load, a transformer having a secondary coil L connected to the terminals of said load and a primary coil L connected to said source, and a variable capacitor K connected across said primary coil and so adjusted as to absorb wattless current drawn by said primary coil, said transformer having a coefficient of coupling it between its coils, said coefficient being of such value as to produce two natural frequencies, one of which is outside the band of frequencies to be amplified, and said capacitors and transformer coils having values so chosen as to obtain a substantially pure input resistance in the network per se, while the conductance of said network has a value where Le=L (17c and L=L'=L", and wherein G is the internal resistance of said source.

5. In a reactance network fed by a source of alternating currents and feeding into a load, such network being constituted by a transformer having its primary shunted by a variable capacitor and its secondary shunted by a second variable capacitor, said transformer having a relatively high coefiicient of coupling, the method of matching the overall impedance of the load and the network to said source of alternating currents which comprises adjusting the second capacitor to a relatively large value, adjusting the first capacitor so as to make the load impedance at substantially pure resistance, then reducing the capaciy of the second capacitor and readjusting the first capacitor in the order named until an efficient transfer of energy is obtained from said source to said load while maintaining said load as a substantially pure resistance.

6. A reactance network fed by a source of alternating current of a given frequency, which source has an internal resistance G, said network feeding into a load resistance R and comprising a transformer having a primary coil connected to said source and a secondary coil connected to said load resistance, the inductances of said primary and secondary coils respectively being of values designated L and L", said transformer having a coefiicient of coupling it between its primary and secondary coils, a capacitor having a capacitance value K connected across said primary coil, another capacitor having a capacitance value C connected across said secondary coil, and said network being so characterized as to satisfy the following equation:

wherein Le=L(1-k where L=L=L", nd wherein G is the aforementioned internal res stance.

WALTER VAN B. ROBERTS. 

